Biaxially-oriented polyester films have advantageous characteristics such as mechanical characteristics, dimensional stability, heat resistance, transparency, and electrical insulating properties and therefore are used for various applications such as magnetic recording materials, packaging materials, electrical insulating materials, a variety of photographic applications, graphic arts, and optical display materials. However, they do not have sufficient surface hardness and therefore have the disadvantage that their surface can be easily damaged by contact, friction, or scratching with other hard materials. In order to solve this problem, conventional techniques use methods of placing various hard coat layers.
Since biaxially-oriented polyester films are highly crystal-oriented, hard coat layers directly provided thereon may have insufficient adhesion. Therefore, methods of providing a hard coat layer on a polyester film through an adhesive layer are generally conducted.
When such methods are used or when a hard coat layer is provided directly on a biaxially-oriented polyester film, a refractive index difference in-plane can be generated between the hard coat layer and the adhesive layer or biaxially-oriented polyester film in contact therewith. Therefore, when such a structure is used as a substrate for optical applications such as antireflection films or touch panel films, interference iris patterns can be generated depending on unevenness in the thickness of the hard coat layer.
In the applications described above, the formation of interference iris patterns significantly inhibits the clear view properties of displays. In order to reduce this phenomenon, the coating thickness accuracy is improved, or the refractive index of the hard coat layer is increased so that the difference between the refractive indices of the hard coat layer and the base film can be reduced (Patent Document 1). Also proposed are a method including the steps of embossing the surface of a base film by hot pressing to form irregularities on the surface and providing a hard coat layer on the embossed surface (Patent Document 2), a method including the steps of using a solvent capable of dissolving a base film to form a hard coat layer-forming coating composition and applying the coating composition to the base film so that the base material can be dissolved or allowed to swell (Patent Document 3), a method of transferring a molding film (Patent Document 4), and a method including the step of adding particles to a hard coat layer to form irregularities for scattering light (Patent Document 5). However, the refractive index of the hard coat layer or the adhesive layer cannot be completely adjusted to about 1.60-1.65, the typical refractive index of biaxially-oriented polyester films, and existing techniques are not satisfactory and cannot completely eliminate interference iris patterns. It is also very difficult to provide a hard coat layer with a uniform thickness such that interference iris patterns can be prevented.
When the method including the steps of forming irregularities on the surface of a base film by hot pressing or the like and providing a hard coat layer thereon is only used, the reflection-suppressing effect cannot be sufficiently achieved, although interference iris patterns can be made slightly less visible. Since biaxially-oriented polyester films have high solvent resistance, few solvents are suitable for the method of dissolving a base film or allowing a base film to swell, and ortho-chlorophenol, the only available solvent, has a problem in which it can pollute the working environment or is not easy to remove. When a mixture layer is formed in the interface between the base film and the hard coat layer, the haze can increase, and the sharpness of the transmitted image from a display using such a technique can be reduced. In this case, since the hard coat layer is made relatively smooth, the external light reflection-reducing effect cannot be expected.
Typical hard coat layers are very smooth. Therefore, when such hard coat layers are subjected to antireflection treatment, the intensity of reflected light from the resulting antireflection layer surface can have a significant wavelength dependency so that a certain color can be strongly visible or color heterogeneity can be caused by unevenness of the antireflection layer coating.
The method of transferring the irregularities of the mold film to the hard coat layer can cause contamination with foreign matter from the mold film and can form irregularities with relatively large slope angles, which can easily cause screen glittering. The method of adding particles to the hard coat layer to form irregularities on the hard coat layer surface can produce the effect of preventing interference iris patterns or reflection but still has a problem in which it can increase screen glittering and therefore reduce clear view properties.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-241527    Patent Literature 2: JP-A No. 08-197670    Patent Literature 3: JP-A No. 2003-205563    Patent Literature 4: JP-A No. 2004-341553    Patent Literature 5: JP-A No. 2003-75604